The present invention relates to a method and apparatus for sealing a mixing vessel. More particularly, the present invention relates to an apparatus for reversibly providing sealing engagement between a vessel and a shaft extending through a wall of the vessel. The invention is useful, for example, for providing a temporary seal during replacement of a mechanical seal, which mechanical seal primarily provides sealing engagement between a mixing vessel and a rotatable shaft.
In material processing equipment such as mixers, it is common that a rotatable shaft is positioned through a wall of a mixing vessel. This penetration is commonly located at the centerline of the top head of the mixing vessel, but may also occur in the sidewall or the bottom head. In a typical mixer assembly, a mechanical seal is employed around the rotatable shaft to close off the annulus surrounding the shaft where the shaft passes through the mixing vessel. These seals prevent the material being agitated from escaping. In addition, the seals prevent any gases that may form during the agitation process from escaping.
A mechanical seal typically includes a stationary member fixed to the seal housing and a mating rotatable member attached to the mixer shaft. Seals are generally considered wearing components in a mixing system and must be replaced relatively frequently. Large hydrostatic pressures that can be present inside the vessels and the replacement of mechanical seals under elevated vessel pressure poses safety risks for maintenance personnel. Replacement of the mechanical seal without an additional means of sealing the vessel can allow dangerous gases to escape and can also allow the shaft to be thrust upwards if the vessel is pressurized and no mechanical provision is in place to retain the mixer shaft.
As a result, replacing mechanical seals typically requires depressurization of the mixing vessel to eliminate the very large hydrostatic thrusts and the possibility of vapor release to the atmosphere present in the mixing vessel. The de-pressurization of the mixing vessel can be undesirable for various reasons. First, the process for reducing vessel pressure can be very time consuming. It takes considerable time to cool the vessel. There are also numerous safety procedures for locking out the mixer drive to prevent operation during maintenance. Proper ventilation of the vessel and elimination of dangerous vapor levels is required before vessel entry is permitted. These can result in hours to days of delay depending on the installation specifics. Eliminating downtime must be kept to a minimum because many times a mixer is critical to the operation of an entire plant and down time represents lost production capacity. Second, certain applications require that the mixing vessel not be de-pressurized because it may cause the loss of valuable product or the manufacturing process may require that the materials being agitated remain under constant pressure. The depressurization of the mixing vessel additionally poses environmental concerns, for example, the emissions escaping from the mixing vessel may violate environmental guidelines and/or the contents of the vessel may pose health risks to maintenance personnel.
Current methods for replacing seals without tank de-pressurization attempt to create a temporary, secondary seal but involve partial disassembly of the mixer and necessary shaft translation to engage the shutoff. In order to allow for shaft translation with the current methods, the bearings have to be disconnected from the shaft. The shaft translation can be intentional as a result of operating the shutoff mechanism or it can be unintentional as a result of the hydrostatic pressure in the vessel acting on an unsupported shaft. Regardless, the translational movement poses safety hazards: gross movement upward or downward can result in physical injury to maintenance personnel and because the mixer is at least partly disassembled, gases can escape during shaft translation, prior to formation of the secondary seal. Further, the translational movement imposes wear on secondary seal components and also exposes those components to dirt and corrosive elements. As a result of wear, dirt, and corrosion, if the shaft does not translate sufficiently to create an effective secondary seal, vapors can escape.
For example, to replace cartridge seals, the mixer is disassembled and shaft translation is used during shutoff to create a secondary seal. As a result, vapors can escape from the tank during the delay in forming the seal while the shaft translates or as a result of incomplete translation due to corrosion and dirt. Further, replacement of cartridge seals typically use a common shaft support ring and shutoff collar which only provide for restrained downward movement and unrestrained upward movement. Such an arrangement does not incorporate a positive static shaft seal, which can be checked for positive closure, and positive locked support.
Accordingly, it is desirable to provide a method and apparatus for effectuating a secondary seal which allows a vessel to remain pressurized during the mechanical seal replacement procedure, and which minimizes fugitive emissions to the atmosphere. It is also desirable to provide a method and apparatus for effectuating a secondary seal that does not involve gross translational movement of the shaft or require the bearing support to be disconnected from the shaft for the shutoff to operate. It is also desirable to have a means to safely confirm that the shutoff has occurred. It is also desirable to have an external means for determining that the shaft is properly supported in the shutoff mode. It is further desirable to provide a method and apparatus for effectuating a secondary seal that minimizes exposure of the secondary seal to wear, dirt, and corrosive elements.
The foregoing needs are met, at least in part, by the present invention where, in one aspect, an apparatus is provided for use with a vessel having a shaft extending therethrough, wherein the apparatus has a stop assembly and a collar which cooperate to minimize translational movement of the shaft during shutoff and to form a reversible seal between the vessel and the shaft. In some embodiments a floating flushing bushing may be incorporated with the apparatus to assist in keeping the apparatus clean.
In another aspect of the invention, a method is provided for minimizing shaft translation during shutoff and for creating a seal in the shutoff position.
The above and other aspects, features and advantages are achieved through the use of novel seal shutoff assemblies as herein disclosed. In accordance with one embodiment of the invention, a seal shutoff apparatus is provided for use with a vessel having a shaft extending therethrough, wherein the seal shutoff apparatus includes a housing having an inner surface and an inward protrusion extending along the entire inner surface of the housing, a collar fixedly disposed on the shaft and protruding into the housing, a hydraulic nut mounted to the housing and a moveable element connected to the hydraulic nut so that the moveable element is axially moveable at least between an operating and shutoff position, wherein the moveable element contacts both the collar and the inward protrusion to form a seal when the moveable element is in the shutoff position.
In accordance with another embodiment of the present invention, the seal shutoff apparatus includes a housing having an inner surface and an inward protrusion extending along the entire inner surface of the housing; a collar fixedly disposed on the shaft and protruding into the housing; a first hydraulic nut mounted to the housing; a second hydraulic nut mounted to the housing; a first moveable element connected to the first hydraulic nut for axial movement at least between an operating position and a shutoff position, wherein the first moveable element is located to one side of the collar and inward protrusion; and a second moveable element connected to the second hydraulic nut for axial movement at least between an operating position and a shutoff position, but located on the opposing side of the collar and inward protrusion, wherein the moveable elements contact both the collar and the inward protrusion to form a seal when the moveable elements are in the shutoff position.
In accordance with yet another embodiment of the invention, a method for forming a reversible seal is provided, wherein the hydraulic nut(s) is(are) actuated to move the shutoff disc from an operating to a shutoff position, resulting in the shutoff disc contacting both the collar and inward protrusion to form a seal.
In yet another embodiment of the present invention, a seal shutoff apparatus for use with a vessel having a shaft extending therethrough, wherein the shaft translates at least between an operating position and a shutoff position, is provided. The seal shutoff apparatus has a stop mechanism and a shaft shutoff collar that cooperate both to constrain translational movement of the shaft and to form a seal between the vessel and the shaft when the shaft is in the shutoff position.
In accordance with yet a further embodiment of the invention, a method of forming a reversible seal is provided, wherein the shaft shutoff collar is moved to the shutoff position causing the collar to contact the stop mechanism thereby creating a seal.
There has thus been outlined, rather broadly, several features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described below and which will form the subject matter of the claims appended hereto.
In this respect, before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and to the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein, as well as the abstract, are for the purpose of description and should not be regarded as limiting.
As such, those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention.